1. Field of the Invention
The present invention relates generally to an apparatus and method for printing paste onto a surface through a screen having a plurality of openings therein, and more particularly to an apparatus for and method of printing solder paste onto a surface of a printed circuit board through the screen that is used in producing surface mount technology circuit boards suitable for use in high technology products.
2. Description of the Prior Art
There are various methods by which solder paste can be printed on a printed circuit board. One approach is to use a well known screen printing apparatus which has a spatula-like squeegee which is arranged to be displaced back and forth along a pair of guide rails and is arranged to be raised and lowered relative to the guide rails. The squeegee is adapted to be in pressure contact with an upper surface of the screen when it is lowered to a printing position where solder paste is printed onto a surface of a printed circuit board supported on a printing table. In this type of apparatus, a considerable amount of solder paste is supplied onto the screen prior to operating the apparatus and the solder paste supplied onto the screen is printed on the surface of the circuit board by reciprocating the squeegee, while its lower end is pressed against the screen to introduce the solder paste into openings of the screen.
Another type of a screen printing apparatus is also well known which comprises a viscous material automatic supply dispenser of the type having a housing of a closed box type in which a chamber for containing the material and a slit-like bottom aperture communicating therewith, having a leading side defined by a front wall and a trailing side defined by a rear wall and being arranged to be normally closed and to be kept open to automatically supply a predetermined amount of the material onto a surface of a board during printing.
In these known screen printing apparatuses, the printing table carrying the circuit board thereon is arranged to be moved between the two predetermined positions,a lower position and a raised printing position where printing of the circuit board occurs. As the paste is printed onto the board through the screen at the printing position, the board is caused to be separated from the lower surface of the screen by moving the printing table downwardly from the printing position to the lower position so that the pastes filled in the screen openings are pulled out and transferred onto the circuit board.
FIGS. 6 and 7 each illustrate a process of separating the circuit board 4 from the screen 5 and the manner in which the solder pastes 8 filled in openings 7 in the screen 5 are pulled out and transferred onto the board in accordance with the known apparatuses and method for printing solder paste onto circuit boards through a screen. Solder paste normally has a high viscosity and when it is printed onto the circuit board 4 through the screen 5 at the printing position, the paste 8 in each opening 7 of the screen 5 is adhered to both inner peripheral surfaces of the opening and the upper surface of the circuit board 4 which is in contact with the lower surface of the screen 5, as shown in FIG. 6(A) and FIG. 7(A).
As the separation proceeds by displacing the printing table downwardly from the printing position, the paste 8 in each opening 7 is gradually extended downwardly in the state in which the central portion of the paste 8 is deformed into concave, as illustrated in FIGS. 6(B).about.(D) and FIG. 7(B). At the same time, the screen 5 held by a screen holder 6 in a tensioned state is downwardly pulled, since it is adhered to the board 4 through the medium of the paste 8, and is caused to be gradually elastically deformed downwards, thereby storing repulsive energy therein (FIG. 7(B)). Then, as their separation proceeds further and the paste in each screen opening 7 is pulled out and transferred onto the board 4, the elastically deformed screen 5 springs up by its repulsive force or spring action and then makes up and down movements or vibrations for a while, as illustrated in FIG. 7(C). These movements or vibrations of the screen 5 tend to make the screen come into contact with the printed paste 8 on the circuit board 4. As a result, as shown in FIG. 6(E) and FIG. 7(D), print pattern of the printed paste is caused to be damaged.
One of other disadvantages indicated with the prior art is that when highly viscous solder paste or a screen having narrow openings therein is used for screen printing, the problem of clogging of the screen openings is experienced from time to time. FIG. 6(E) shows one example where a screen opening 7 is partly clogged by solder paste 8.